I've been having performance issues with Flash in Firefox on Windows 8 ever since I installed it, so I decided to try and get to the bottom of the issue, since Chrome's Pepper Flash plugin works flawlessly on the same machine. I decided to install Windows 7 as a dual-boot environment to do some testing, and installed the same version of Firefox (21), with all the same extensions, the same version of Flash (11.7) and the same display driver (Catalyst 13.1 Legacy). I was surprised to find that the performance issues I'd been having in Windows 8 were completely gone.
On closer inspection, that seems to be because the 'Use hardware acceleraton' checkbox is doing nothing in Windows 8. Whether it's checked or not, playing a beefy Flash video (one of Giant Bomb's HD progressive streams) uses 35-40% CPU and is pretty choppy. The exact same video, delivered via the exact same software but under Windows 7 is entirely smooth and CPU usage is around 5-10% with the checkbox ticked, and comparable to Windows 8 with hardware acceleration turned off.
In another test, I compared videos on Youtube with the information box turned on. In Windows 7 it's showing as software video rendering and accelerated video decoding regularly, changing to accelerated for both in full screen. With the exact same setup on Windows 8, it shows software rendering and decoding no matter what, with the associated higher CPU use and poorer video performance. Incidentally, reverting to Flash 10.3 on Windows 8 completely fixes the problem. Hardware acceleration works as it should in all scenarios. It seems to just be the 11 series that's affected.
Is this a known issue at all, and is there any sort of fix planned? Or is there a way to fix it now? Any help would be appreciated.